Overcoming charge transfer barriers via electrostatically stabilized CsPbBr3 nanocrystals for efficient perovskite light-emitting diodes

Electrically insulating organic ligands with long hydrocarbon chains cause bottlenecks, which hinder efficient charge injection and transportation in CsPbBr3 nanocrystal (NC)-based light-emitting diodes (LEDs). Shorter ligand exchange with precise control of the surface ligand density through additional purification processes is a common strategy used to improve charge transport in optoelectronics. However, practical applications have been limited owing to poor structural integrity and colloidal stability associated with low steric hindrance of short surface-capping ligands. This study reports a post-synthesis treatment on CsPbBr3 NCs using hydrazine monohydrobromide (N2H5Br; HZBr) containing NH3+ cations, NH2 groups, and Br- anions. Despite their short molecular structure, hydrazinium ligands produce a net positive charge at the NC surface, enhancing the colloidal dispersion of the CsPbBr3 NCs through electrostatic stabilization. Cryogenic photoluminescence spectroscopic analysis identified that the exciton-phonon coupling at the surface of CsPbBr3 NCs is significantly reduced by the presence of HZBr, preventing the formation of both permanent and transient exciton traps. Furthermore, enhanced surface stabilization to improve the radiative recombination was accomplished by excess Br- anions passivating Br- ion vacancies at the CsPbBr3 NC surfaces. LEDs based on the HZBr-treated CsPbBr3 NCs showed improved current efficiency of 12.16 cd/A, and the external quantum efficiency of 3.92% increased by a factor of 1.8, indicating enhanced hole-electron injection and transport into the CsPbBr3 NCs.